DP Vs. Electronic Flow Switch

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Designer69

Senior Member
Gents,

We are looking at replacing a DP flow switch (which we currently use for low flow alarm / pump protection) with an electronic flow switch.

the reason is that the vendor actually warned us that if the DP switch is running pegged out for a long amount of time (ie, 2 years) it can not be relied on to operate is intended should a low flow condition occur. Meaning the switch could get stuck in that position and fail to close.

My question is... what are some of the characteristics, positives/negatives, features etc of a DP device vs. electronic device?

Thanks
 
GeorgeB

GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
Designer69 said:
Gents,

We are looking at replacing a DP flow switch (which we currently use for low flow alarm / pump protection) with an electronic flow switch.

the reason is that the vendor actually warned us that if the DP switch is running pegged out for a long amount of time (ie, 2 years) it can not be relied on to operate is intended should a low flow condition occur. Meaning the switch could get stuck in that position and fail to close.

My question is... what are some of the characteristics, positives/negatives, features etc of a DP device vs. electronic device?

Thanks
Click to expand...
I'm a hydraulics guy and I'll give you the issues WE face.

If a spool (dP or solenoid valve, wouldn't matter I don't think) is held in one position, silt builds and the spool often will not shift (or spring return, if shifted for a long period).

For valves, we ask that they be shifted at least once a day, generally. For (spool or piston) pressure switches, I've seen them stick and never thought about this as the same problem before. Today, modern electronics and pressure transducer technology are replacing the mechanical switches in many applications.

Not exactly what you asked, but something.
 
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Designer69

Senior Member
this is used for low flow protection of a feedwater pump and located on the feedwater pipe, suction side of the pump.

Thanks
 
P

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Designer69 said:
this is used for low flow protection of a feedwater pump and located on the feedwater pipe, suction side of the pump.

Thanks
Click to expand...

That tells me how it is being used, not what technology is being used to sense flow. There are a bunch such as D/P, conductivity, thermal dispersion, vane, etc.
 
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BJ Conner

Senior Member
Location
97006
DP switch where?

DP switch where?

Designer69 said:
sorry, it is a DP switch
Click to expand...

DP = differentical pressure switch. Differentical accross what? an orrifice? the pump? a valve? A flange in a pipe joint?
If you want to verify flow I would use an orrifice with a differential pressure switch across the orffice. The orifface can be large, your not measuring anything so you just want to generate a DP. Paddle switches have been known to have parts break off and go into pumps.
 
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markstg

Senior Member
Location
Big Easy
Designer69 said:
Gents,

We are looking at replacing a DP flow switch (which we currently use for low flow alarm / pump protection) with an electronic flow switch.

the reason is that the vendor actually warned us that if the DP switch is running pegged out for a long amount of time (ie, 2 years) it can not be relied on to operate is intended should a low flow condition occur. Meaning the switch could get stuck in that position and fail to close.

My question is... what are some of the characteristics, positives/negatives, features etc of a DP device vs. electronic device?

Thanks
Click to expand...

Sorry I'm not going to answer your question, but give you an alternative method that I believe is more reliable.

Switches are being used less and less because if they are not tested periodically you don't really know if it will actuate when called upon. Mechanical switches (paddle, diaphragm, bellows) can just get cruded up or permanently deformed.

I would consider a dp transmitter with an integral indicator. By having a continuous signal you know if the signal is alive by the small flucuations in dp. Connect the signal to a currrent relay which is all electronic and are very reliable.

If you have to stay with a switch no matter what, you may consider a thermal flow switch. Maybe they have a diagnosic led that lets you know all is well of if the sensor is cruding up.
 
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Designer69

Senior Member
i guess the isssue is that I don't really understand deadband and reset of differential pressure switches.

i would really appreciate an explanation, thanks
 
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markstg

Senior Member
Location
Big Easy
Designer69 said:
i guess the isssue is that I don't really understand deadband and reset of differential pressure switches.

i would really appreciate an explanation, thanks
Click to expand...

Deadband may best be understood with an example:

Switch Trip Point setting = 10 psi decreasing
DeadBand = 2 psi

As pressure decreases below 10 psi switch changes state, as pressure increases from below 10 psi, the switch will not change state (reset) until 12 psig the deadband value. Deaband prevents switches from chattering aroung the trip setting.
 
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dicklaxt

Senior Member
Designer69,markstg's recommendation for a dp transmitter/current relay is no doubt the best solution,,,,,,,,,,out of curiosity is the present system of alarm and shutdown done via a PLC,DCS etc or is it hard wired relay logic?

marcstg,does the dp switch with deadband you described have more than one switch or do you have to multiply contacts by otgher means for multifunction scenarios?

dick
 
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markstg

Senior Member
Location
Big Easy
dicklaxt said:
Designer69,markstg's recommendation for a dp transmitter/current relay is no doubt the best solution,,,,,,,,,,out of curiosity is the present system of alarm and shutdown done via a PLC,DCS etc or is it hard wired relay logic?

marcstg,does the dp switch with deadband you described have more than one switch or do you have to multiply contacts by otgher means for multifunction scenarios?

dick
Click to expand...

You can get a switch with spdt, dpdt contacts, and you can also get switches with 2 set points....use as a hi and lo, or a hi and hi hi, or lo and lo lo.

I'm fortunate to work where all we have is PLC's and DCS so usually don't need relays to interlock with.

An idea for being able to test a switch on-line, it to provide a bypass switch, perhaps a key switch for only authorized access, and bypass the switch, take the switch and test it so see it functions, then put it back in service or replace it.
 
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Designer69

Senior Member
it is just done by relays. when the flow switch contacts close, it is connected to a time delay relay which after 15 seconds energizes a breaker tripping device which opens the breaker and shuts off the feedwater pump.
 
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Designer69

Senior Member
markstg,

since you are now the forums flow switch guru, can you get DP flow switches that work on increasing and decreasing flow or only one or the other but not both?

thanks
 
M

markstg

Senior Member
Location
Big Easy
Designer69 said:
markstg,

since you are now the forums flow switch guru, can you get DP flow switches that work on increasing and decreasing flow or only one or the other but not both?

thanks
Click to expand...

Thanks, and yes both directions can be in one housing. That's what I meant above by Hi and Lo switch.

And DP switches are not just for flow applications, DP across filters is another application.
 
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IslandSparky

Member
I prefer to have analog pressure transducers with external logic to trigger the action. You can then test the output against anticipated real-time results if things are integrated in a plc, or just create your own filtering on the output. It is more expensive, but you gain a lot of flexibility.
 
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